Spark gap device



March 25, 1958 7R. E. MARBURY $828,436

SPARK GAP DEVICE Filed Feb. 8, 1954 INVENTOR Ralph E.Morbury.

ATTORNEY SPARK GAP DEVICE Ralph E. Marbury, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 8, 1954, Serial No. 408,821

6 Claims. (Cl. 313-185) The present invention relates to spark gap devices, and more particularly to a sealed, low-pressure gap device having low and consistent breakdown voltage.

Spark gap devices are frequently used for overvoltage protection, the gap device being connected across the pro tected equipment so that it will break down and bypass the protected equipment upon the occurrence of apredetermined overvoltage. In many cases such a protective gap must have a relatively low breakdown voltage, of the order of a few hundred volts, for example, in order to provide adequate protection. Such low breakdown voltages can be obtained with gaps at atmospheric pressure having very short gap spacing, or with sealed, lowpressure gaps having longer gap spacing. Gaps operating at atmospheric pressure are very difficult to build and calibrate for low breakdown voltage, because the necessary spacing of the electrodes is very small and is very critical, and it is difficult to maintain the calibration unchanged after repeated operations. Sealed low-pressure gaps are therefore preferable where low breakdown voltage is required, if such gaps can be built with accurate and consistent breakdown voltage.

It has been found, however, that low-pressure gap devices frequently have relatively high and inconsistent initial breakdown voltage, probably due to the difiiculty of starting ionization because of the low gas pressure in the gap. That is, when an excess voltage is first applied to such a gap, the voltage at which breakdown occurs is likely to be quite high and may vary unpredictably within rather wide limits on gaps of the same design, or on different tests of the same gap, although after the first breakdown successive tests at short intervals show low and quite consistent breakdown voltages. Obviously adequate overvoltage protection cannot be reliably obtained with such a gap, since long periods of time may elapse between operations, and the breakdown voltage may thus be dangerously high when an overvoltage does occur and the gap is expected to operate.

The principal object of the present invention is to provide a low-pressure spark gap device which has a consistent and reliable initial breakdown voltage, so that it can be used for overvoltage protection with safety and reliability.

Another object of the invention is to provide a lowpressure spark gap device in which means are provided for starting ionization in the gap to facilitate breakdown, so that low and consistent breakdown voltages are obtained.

More specifically, a spark gap device is provided in which an insulating member of material of high dielectric constant extends between the electrodes of the gap closely adjacent the actual gap space to efiect preionization of the gap, so as to facilitate breakdown and obtain low and consistent breakdown voltage.

Other objects and advantages of the invention will be apparent from the following detailed description, taken in connection with the accompanying drawing, the single Sttes atent 2,828,436 Patented Mar. 25, 1958 figure of which is a longitudinal sectional view of a gap device embodying the invention.

The invention is shown in the drawing embodied in a low-pressure spark gap device of a type which is suitable for overvoltage protective applications, such,as for the protection of capacitors connected in series in a distribution line for improving the voltage regulation, forexample, or other similar applications. In the preferred embodiment of the invention shown in the drawing, the gap is contained in a sealed envelope or enclosure 1 which may be made of glass, and which is preferably provided with a stem or tubulation 2 for evacuating the envelope.

The gap device has two generally cylindrical electrodes generally designated 3 and 4, which are disposed coaxially in the envelope 1. The electrode 3 includes a tubular electrode member 5 of copper, or other suitable conduct-' ing material, which is closed at one end by a disc 6 which may also be of copper and which may be brazed to the tubular member 5. A stem 7 is secured to the disc6 and extends through the envelope 1 and is sealed therein, as indicated at 8, the stem 7 preferably being made of a conducting material which is suitable for forming a permanent air-tight seal with the glass of the envelope 1.

The electrode 4 comprises a solid cylindrical copper electrode member 9 with a stem 10, preferably of the same material as the stem 7, secured in the electrode member 9. The stem 10 extends through the envelope 1 and is sealed therein, as indicated at 11.

The electrodes 3 and 4 are disposed coaxially in theenvelope 1, with the electrode member 9 extending into the tubular electrode member 5 to form a radial gap space 12 between the electrodes. After assembly of the electrode members, the envelope 1 is evacuated to a pressure substantially less than atmospheric, the exact pressure being determined by the desired breakdown voltage, and the stem 2 is sealed off.

As previously explained, low-pressure gaps of the conventional types have high and inconsistent initial breakvoltages. In accordance with the present invention, this difficulty is overcome, and low and consistent breakdown voltages are obtained, by means of an insulating member 13 disposed between the electrodes. The insulating member 13 is made of a material having a high dielectric constant, a suitable material for this purpose being a ceramic material made of rutile, which 'is a natural mineral consisting chiefly of titanium dioxide. This material has a very high dielectric constant, which may, for example, be of the order of 100, and is very suitable for the purpose although it will be understood that other suitable insulating materials may be used, and in general any material having a dielectric constant substantially higher than that of air may be used. l

The insulating member 13 is disposed between the electrodes to substantially bridge the space between them closely adjacent the actual gap space 12, and may be supported in'position in any suitable manner. In the preferred embodiment shown in the drawing, the insulating member 13 has a central opening and is supported on the electrode member 9 by means of a hollow cup-shaped extension 14 on the electrode member whichexten'ds through the central opening of the insulating member 13 and is spun over to hold the insulating member firmly in place on the electrode member 9. The insulating metro ber 13 is circular in shape and has an outside diameter only slightly less than the inside diameter of the tubular electrode member 5, so that the insulating member extends across the space between the electrode members closely adjacent the gap space 12 and substantially bridges the space between the electrodes.

When a voltage is applied across two dielectric materials in series, the voltage distribution between the two materials is inversely proportional to their dielectric constants. In the gap construction described, when voltage is applied across the electrodes 3 and 4, the voltage is also applied across the insulating member 13 in series with a very short air gap, or thin film of air, between the insulating member 13 and electrode 5. Since the insulating member 13 has a high dielectric constant, as compared to that of the air, the voltage will be concentrated across the air gap, and since this air gap is extremely short, the voltage gradient will be very high. As the voltage across the electrodes 3 and 4 is raised, the air in the narrow space between the insulating member 13 and the electrode 5 will be locally overstressed and ionization will occur. This results in a substantial amount of ionization in the gap space 12, so that the gap is preionized and breakdown of the gap is facilitated. A low breakdown voltage is thus obtained which is always consistent, whether or not the gap has recently discharged, since adequate ionization is always present in the gap space when the voltage approaches breakdown. The arrangement of the device must, of course, be such that the short gap between the insulating member 13 and the electrode 5 is near the main gap 12 and is not screened from it, so that the ions can move freely into the main gap.

It should now be apparent that a sealed, low-pressure spark gap device has been provided in which the breakdown voltage is always uniform and consistent, so that the device is very well suited for overvoltage protective applications. A preferred embodiment of the invention has been shown and described for the purpose of illustration, but it will be obvious that various modifications may be made within the scope of the invention, and that the invention is not limited to the specific details of construction shown but includes all equivalent modifications and embodiments.

I claim as my invention:

1. A spark gap device comprising two electrodes disposed in a sealed envelope and spaced apart a distance suflicient to provide a spark gap between them, the envelope being evacuated to a pressure substantially less than atmospheric, and a member of insulating material of high dielectric constant disposed between the'opposing surfaces of the electrodes and leaving a small gap between the said insulating member and at least one of said electrodes, said insulating members being disposed to effect ionization of the spark gap when a voltage approaching the breakdown voltage of the spark gap is applied across the electrode members.

2. A spark gap device comprising two electrodes disposed in a sealed envelope and spaced apart a distance sufiicient to provide a spark gap between them, the envelope being evacuated to a pressure substantially less than atmospheric, and a member of insulating material of high dielectric constant disposed between the opposing surfaces of the electrodes supported on one of the electrodes and extending across the space between the electrodes leaving a small gap between the said insulating member and the other of said electrodes, said insulating members being disposed to effect ionization of the spark gap when a voltage approaching the breakdown voltage of the spark gap is applied across the electrode members.

3. A spark gap device comprising two generally cylindrical electrode members disposed coaxially within a sealed envelope, one of said electrode members being hollow and the other electrode member extending within the hollow electrode member and being spaced therefrom a distance sutlicient to form a spark gap, said en velope being evacuated to a pressure substantially less than atmospheric, and a member of insulating material of high dielectric constant disposed between the opposing surfaces of the electrodes and leaving a small gap be} tween the said insulating member and at least one of said electrodes, said insulating members being disposed to effect ionization of the spark gap when a voltage approaching the breakdown voltage of the spark gap is appiied across the electrode members.

4. A spark gap device comprising two generally cylindrical electrode members disposed coaxially within a sealed envelope, one of said electrode members being hollow and the other electrode member extending within the hollow electrode member and being spaced therefrom a distance sufiicient to form a spark gap, said envelope being evacuated to a pressure substantially less than atmospheric, and a member of insulating material of high dielectric constant disposed between the opposing surfaces of the electrodes supported on one of the electrode members within the hollow electrode member and extending across the space between the electrodes leaving a small gap between the said insulating member and at least one of said electrodes, said insulating members being disposed to effect ionization of the spark gap when a voltage approaching the breakdown voltage of the spark 7 of high dielectric constant disposed between the opposing surfaces of the electrode members leaving a small gap between the said insulating member and at least one of said electrodes, said insulating members being disposed to effect ionization of the spark gap when a voltage approaching the breakdown voltage of the spark gap-is applied across the electrode members.

6. A spark gap device comprising a tubular electrode member and a generally cylindrical electrode member disposed coaxially within a sealed envelope, the cylindrical electrode member extending into the tubular electrode member and being spaced therefrom a distance sufficient to form a radial work gap therewith, said envelope being evacuated to a pressure substantially less than atmospheric, and a member of insulating material of high dielectric constant disposed between the opposing surfaces of the electrodes supported on one of the electrode members within the tubular electrode member leaving a small gap between the said insulating member and the other electrode member, said insulating member being generally cylindrical and being disposed to eliect ionization of the spark gap when a voltage approaching the breakdown voltage of the spark gap is applied across the electrode members.

References Cited in the file of this patent UNITED STATES PATENTS 

